Okay i see, M_PK_Default_Decal_AlphaTest and M_PK_Default_Decal_Blend are both made to be used for pure decal, and not on particle sprite. (Usage for particle sprite is not checked).
For what i understand for now, this is both because of us, and because of the way UE render decals. Try to activate the "DBuffer Decals" option if not done yet in your engine settings. (you can also open DefaultEngine.ini and set to "True" the r.DBuffer option).
However, we should be able to handle this situation, and it should not crash because of one of our material.
If you use only static light in your scene, take a look at this page : http://www.worldofleveldesign.com/categories/ue4/ue4-decals03-decals-do-not-render-in-indirect-light.php
(it may help)
I will look deeper next week this issue and fix it asap.
Please let me know if you found any workaround, or solution, for your problem.